Aging is associated with a decline in adaptive immunity that leaves senior citizens with decreased responses to vaccines and increased vulnerability to disease from infectious agents. Much of the decline is found in the B cell arm of immune responses, leaving the elderly with a suboptimal antibody response and increased vulnerability to disease. Rabbits provide a useful model for studying the decline in B lymphopoiesis because it declines precipitously a few weeks after birth, and by 2 to 4 months of age, little or no B lymphopoiesis occurs. The decline in B lymphopoiesis could be due to intrinsic changes in early B lymphocyte precursors, to changes in the bone marrow (BM) stromal environment, or to feed-back regulation by peripheral B cells. The BM becomes highly adipose as does human BM and we recently discovered that both human and rabbit adipocytes secrete a molecule that inhibits B-lymphopoiesis at the CLP to pre-pro-B cell stage. The goal of this grant is to elucidate the mechanism by which B lymphopoiesis arrests and discover a means to re-initiate production of new B cells in vivo. In Aim 1, we will compare the B-lymphopoietic capacity of early B-cell progenitors in BM of young and adult rabbits as well as the capacity of osteoblasts to support the differentiation of early B-lineage cells. In Aim 2, we will isolate and characterize the inhibitor secreted by adipocytes and also determine the mechanism by which the adipocyte-derived factor inhibits B-lymphopoiesis. We will determine if the inhibitor acts on early B cell progenitors or on the stromal cells and investigate changes in gene expression induced by the inhibitor. In Aim 3, we will focus on re-initiating B lymphopoiesis by inhibiting adipogenesis with statins, by promoting generation of MSC and osteoblasts with parathyroid hormone, and by depleting peripheral B cells. We will perform a retrospective study of BM transplant patients to determine if patients on statins engraft more successfully than age-match controls. Results from studies in this proposal will lead to future experiments in humans to identify mechanisms that contribute to the decline in adaptive immunity in the elderly, as well as a means by which this can be reversed, leading to improved responses to vaccines.